Compression ignition engines or diesel engines rely on the pressure and resultant temperature of the fuel in the cylinder in order to cause ignition to drive the engine. As is well known, in each cylinder it is necessary to provide a glow plug to raise the temperature of the fuel during cold starts and other conditions when the fuel and environmental temperatures are low. Glow plugs are typically wire wound devices having a very low resistance. These devices are electrically connected through a controller across the vehicle batteries drawing heavy current loads. The reason for the low resistance is to generate a high temperature in a short response time.
Controllers for wire wound glow plugs contain one or more relays and one or more relay contacts in the circuit in order to open and close the power path to the glow plug. This opening and closing operates to regulate the amount of current flowing to the glow plug as well as turning the glow plug off when the temperature of the engine is sufficient for compression ignition.
Wire wound glow plugs are now being replaced with surface film glow plugs wherein a predetermined temperature coefficient heating material, such as a positive temperature coeffieicnt material is deposited on a ceramic base. This glow plug is then positioned in the cylinder in a manner similar to its wire wound predecessor. The resistance value of the heating material on the glow plug is generally higher than that of the wire wound on the glow plug, however, the heating time of the heating material is much faster than the wire wound. In order to accurately control the heating of the surface film glow plugs, it is necessary to replace the relays and the several contacts with faster acting solid state components.